A library approach to operating system (OS) construction was championed by several operating system designs in the 1990s. The idea of the library OS is that the entire personality of the OS on which an application depends runs in its address space as a library. An OS personality is the implementation of the OS's application programming interfaces (APIs) and application visible semantics—the OS services upon which applications are built. Early proponents of the library OS approach argued primarily that the library OS could enable better performance through per-application customization. For example, a disk-I/O bound application with idiosyncratic file access patterns can realize better performance by using a custom file-system storage stack rather than using the default sequential prefetching heuristics.
Like many of its contemporaries, the library OS approach is largely forgotten, a casualty of the rise of the modern virtual machines. While most new OS designs of the time, including library OS design, run only a handful of custom applications on small research prototypes, virtual machine systems proliferated because they could run major applications by reusing existing feature-rich operating systems. The performance benefits offered by library OS designs did not overcome the need for legacy compatibility.